[clarkss12]

Thanks, a little hesitant after reading that. Main reason for interest is being able to reboot to Android OS side w/a nice GUI than the tablet-based one Ugoos has for primarily SmartTube (YouTube). I can never get YT to work via the Kodi add-on. Think @clarkss12 has installed SlimBOXtv on several of his Android boxes & can comment.

Apparently that is one of the few media players that I have not installed the Slimbox firmware on, at least as I recall.  I just checked and it is running the Ugoos UI and not slimbox UI.  
I know you have already visited this link, but for others looking for that firmware.

https://slimboxtv.ru/ugoos-am6-pro-plus/

 As you are aware, I have swapped firmware's for years, so I am not hesitant to do that.   It is really hard to brick one of these media players by swapping firmwares.

[hdmkv]

Thanks... I'll try it on one my my 2 AM6B+ units.

[Sholander]

Can't you just use another launcher that suits your GUI taste? On ALL my boxes that I use Android I install "ATV Launcher Pro", and set it the same across all boxes.
I use it on regular (tablet) versions as well as on ATV ones. BTW, on AM6B+ it's very easy to swap launchers...

[hdmkv]

That's a good idea, or using Projectivy.